With the development of technology, traditional display devices and touch panels could not meet requirements of consumers. Therefore, flexible electronic products are developed. Firstly, a basic requirement for materials of display panels and touch panels is that the material has good optical transmittance, which makes contents displayed on the electronic products clear to users. As a result, a flexible transparent substrate is required to have optical transmittance over 80% in visible range.
Secondly, because the traditional display devices and the touch panels are not flexible, glass substrates are good enough to meet the requirement of optical transmittance. However, the glass substrate is thick, heavy and fragile so that another kind of substrate is developed to replace the glass substrate for being flexible and thinner. In addition, flexible electronic products require flexible transparent substrates. Plastic substrates which are flexible and have high transmittance are in the limelight in the field.
In addition, since a conducting layer need to be set on transparent plastic substrates so that the transparent plastic substrates need to endure the change of temperature during a manufacturing process of semiconductors of the display panels and the touch panels. To meet this requirement, the transparent plastic substrates are designed to have good heat endurance. Generally speaking, the transparent plastic substrates must endure the temperature above 250° C. such that there is no damage on the transparent plastic substrates during the process.
Polyimide thin film has good characteristics of flexibility, lightness, heat endurance and is widely used in semiconductor products. However, owing to the charge transfer complex effect of polyimide thin film, the color of the polyimide becomes yellow or red-brown. The color transition is unfavorable to the substrate of the display panels and touch panels.
However, when the polyimide thin film are utilized in the flexible display panels and touch panels of flexible electronic products, polyimide thin film has a trilemma that good optical transmittance, flexibility and heat endurance cannot maintain at the same time. Thus, manufactures try to develop the polyimide to have good transmittance, good flexibility and high heat endurance at the same time.